1. Field of the Invention
The present invention relates to a motor employed in a disc drive such as a hard disc drive (HDD), as a drive source of the disc drive for driving a disc such as a compact disc (CD), a compact disc read only memory (CD-ROM) or a digital versatile disc (DVD).
2. Description of the Related Art
Recently, there is a spindle motor with a fluid dynamic bearing in a dynamic pressure type as a rotary drive source for a disc which is an information record medium. The spindle motor is configured to firmly attach a sleeve on a motor base which forms a stator and to fill lubrication oil in a gap between the sleeve and a rotary shaft. The spindle motor has the following advantageous features: (1) the load facility is large; and (2) the high-speed capability is excellent.
A bearing structure of the spindle motor is disclosed in Japanese Patent Application Laid-Open No. 2004-328926. In the bearing structure, a thrust bearing portion where a dynamic pressure is to be generated is provided between the sleeve and a rotor hub, and a retaining mechanism for a rotor is provided by forming a flange portion on an outer circumference of the sleeve. However, even if the thrust load is supported at the thrust bearing portion, there is a problem that the bearing structure is weak with respect to thrust force acting in a direction in which the rotor comes out of the spindle motor because force acting against the direction in which the rotor comes out of the spindle motor is only magnetic force of a magnet.
In order to resolve the above problem, another bearing structure is disclosed in Japanese Patent Application Laid-Open No. 2006-183734. In the bearing structure, two thrust bearing portions are provided to support thrust load which is generated by a dynamic pressure at the two thrust bearing portions and acts in two directions along an axial direction.
As shown in FIG. 1, a rotor hub Rh is formed in a cupped shape. The rotor hub Rh has an annular wall portion W at an outer periphery portion thereof and a center portion Rs1 at a center portion thereof. A rotor spindle Rs is composed of the center portion Rs1 and an outer cylinder portion Rs2. The center portion Rs1 is integrally formed in the rotor hub Rh. The outer cylinder portion Rs2 is fitted to an outer periphery portion of the center portion Rs1. A sleeve S supports the rotor spindle Rs and has a flange portion f at one end side of an outer periphery portion thereof. A seal ring Sr is fixed at the other end side of the outer periphery portion of the sleeve S. A thrust ring Tr is inserted between the flange portion f and the seal ring Sr. The thrust ring Tr is fixed in the annular wall portion W and abuts on a step portion of the annular wall portion W at an outer periphery portion thereof. In the rotor hub Rh, lubrication oil is filled in gaps between the flange portion f and the annular wall portion W, between the flange portion f and the thrust ring Tr, between the sleeve S and the thrust ring Tr, between the thrust ring Tr and the seal ring Sr, and between the seal ring Sr and the annular wall portion W. A taper seal portion Ts is formed at a gap between the seal ring Sr and the annular wall portion W to prevent the lubrication oil from leaking outside. Thrust bearing portions TB and TB each where a dynamic pressure of the lubrication oil is to be generated are formed between the flange portion f and the thrust ring Tr and between the seal ring Sr and the thrust ring Tr, respectively.
In the above-described bearing structure, it is preferred that the taper seal portion Ts is located in the vicinity of a center area of the bearing structure. In this configuration, even if the lubrication oil spreads outside due to centrifugal force, it is not likely that the spreading lubrication oil arrives at an outside of the rotor hub Rh. This provides a large space for motor drive members such as a field magnet Mg and a drive coil C to be disposed in the outside of the rotor hub Rh, in the bearing structure.
However, in reality, it is impossible to locate the taper seal portion Ts in the vicinity of the center area of the bearing structure because the taper seal portion Ts should be disposed in an outside of the flange portion f due to the bearing structure. In a case where an engineer wants to force the taper seal portion Ts to be located in the vicinity of the center area of the bearing structure, lengths of the thrust bearing portions TB and TB should be reduced as sacrificing characters of the thrust bearing portions TB and TB.
The above-described bearing structure also requires high impact resistance. However, the thrust ring Tr is flattened in shape and pressure-inserted into a part of the annular wall portion W to be held only at the outer periphery portion thereof. This can not provide the bearing structure sufficient pressure-insertion intensity. If external force acts against the rotor spindle Rs in a direction (upper direction in FIG. 1) where the rotor spindle Rs moves away from the sleeve S, it is likely that the external force misaligns a relative position between the rotor hub Rh and the thrust ring Tr.
When the above-described bearing structure is assembled, the sleeve S is loosely fitted to an inner hole of the thrust ring Tr and then pressure-inserted into an inner hole of the seal ring Sr. Then, this assembled bearing unit is pressure-inserted into the annular wall portion W. At this time, the assembled bearing unit is pressure-inserted by pressing the seal ring Sr against the annular wall portion W. In this process, it is likely that each thrust bearing portion TB is flattened out. If the thrust bearing portion TB is flattened out, the bearing structure can not obtain dynamic pressure requirement.
In a case where the field magnet Mg is fixed to the bearing structure and the drive coil C is disposed at a stator side which is an outside of the field magnet Mg, one or more members located in an inner hole of the field magnet Mg needs to be made of a magnetic material in order to form a magnetic circuit in the inner hole. In the above-described bearing structure, the whole of rotor hub Rh is made of a magnetic material because the annular wall portion W is located in the inner hole of the field magnet Mg. This reduces drive torque to be generated in the drive coil C because a part of magnetic fluxes is introduced into a rotor spindle Rs side to reduce magnetic flux acting at a drive coil C side.